Until recently, display devices have typically used cathode-ray tubes (CRTs). More recently, considerable effort has been expended to research and develop thin film transistor liquid crystal display (TFT-LCD) devices having thin profiles, light weight and low power consumption as substitutes for CRTs.
Liquid crystal display (LCD) devices use the optical anisotropy and polarization properties of liquid crystal molecules of a liquid crystal layer to produce an image. The liquid crystal molecules have long and thin shapes. Because of the optical anisotropy property, the polarization of light varies with the alignment direction of the liquid crystal molecules. The alignment direction of the liquid crystal molecules can be controlled by varying the intensity of an electric field applied to the liquid crystal layer. Accordingly, a typical LCD device includes two substrates spaced apart and facing each other and a liquid crystal layer interposed between the two substrates. Each of the two substrates includes an electrode on a surface facing the other of the two substrates. A voltage is applied to each electrode to induce an electric field between the electrodes. The arrangement of the liquid crystal molecules as well as the transmittance of light through the liquid crystal layer is controlled by varying the intensity of the electric field. LCD devices are non-emissive type display devices that employ a light source to display images using the change in light transmittance.
Among the various types of LCD devices, active matrix LCD (AM-LCD) devices that employ switching elements and pixel electrodes arranged in a matrix structure are the subject of significant research and development because of their high resolution and superior suitability for displaying moving images. Thin film transistor LCD (TFT-LCD) devices use thin film transistors (TFTs) as the switching elements.
FIG. 1 is a perspective view of an LCD device according to the related art. As shown in FIG. 1, the LCD device of the related art includes a first substrate 10, a second substrate 20 and a liquid crystal layer 30. The first substrate 10 is referred to as an array substrate and includes a gate line 14 and a data line 16 crossing each other to define a pixel region P. A pixel electrode 18 and a thin film transistor (TFT) Tr, as a switching element, are positioned in each pixel region P. Thin film transistors T, which are disposed adjacent to crossings of the gate lines 14 and the data lines 16 are disposed in a matrix on the first substrate 10. The second substrate 20 is referred to as a color filter substrate, and includes color filter layer 26 including red (R), green (G) and blue (B) color filters 26a, 26b and 26c, a black matrix 25 between the red, green and blue color filters 26a, 26b and 26c and a common electrode 28 on both the color filter layer 26 and the black matrix 25.
Although not shown in FIG. 1, the first and second substrates 10 and 20 are attached with a seal pattern to prevent leakage of liquid crystal layer 30. In addition, a first alignment layer is formed between the first substrate 10 and the liquid crystal layer 30 and a second alignment layer is formed between the second substrate 20 and the liquid crystal layer 30 to align the liquid crystal molecules in the liquid crystal layer 30 along an initial alignment direction. A polarization plate is formed on an outer surface of at least one of the first and second substrates 10 and 20.
Further, a backlight unit (not shown) disposed under the first substrate 10 supplies light. A gate signal turning the TFT Tr on is sequentially applied to each of the gate lines 14, and an image signal on the data line 16 is applied to the pixel electrode 18 in the pixel region P. The liquid crystal molecules in the liquid crystal layer 30 are driven by a vertical electric field generated between the pixel electrode 18 and the common electrode 28 to display images by varying the light transmittance of the liquid crystal molecules.
FIG. 2 is a cross-sectional view illustrating an array substrate for an LCD device according to the related art. In FIG. 2, a gate electrode 60 is formed on a substrate 59 having a pixel region P defined by a gate line (not shown) and a data line (not shown). A gate insulating layer 68 is formed on the gate electrode 60, and a semiconductor layer 70 including an active layer 70a and an ohmic contact layer 70b is formed on the gate insulating layer 68 over the gate electrode 60. Source and drain electrodes 76 and 78 spaced apart from each other are formed on the ohmic contact layer 70b. After the semiconductor layer 70 is patterned through a single mask process, a metallic material layer on the semiconductor layer 70 is patterned through a different mask process to form the source and drain electrodes 76 and 78. As a result, the source and drain electrodes 76 and 78 completely cover end portions of the semiconductor layer 70. In addition, a passivation layer 86 is formed on the source and drain electrodes 76 and 78. The passivation layer 86 includes a drain contact hole 80 exposing the drain electrode 78. A pixel electrode 88 is formed on the passivation layer 86 in the pixel region P. The pixel electrode 88 is connected to the drain electrode 78 through the drain contact hole 80.
Patterns of an array substrate for an LCD device are formed through a photolithographic process using a photoresist (PR) of a photosensitive material. The photolithographic process includes a step of coating a PR on one of a metallic material layer, an insulating material layer and a semiconductor material layer, a step of exposing a PR layer through a photo mask, a step of developing the exposed PR layer to form a PR pattern, a step of etching the one of the metallic material layer, the insulating material layer and the semiconductor material layer using the PR pattern as an etch mask to form a line, an electrode, a contact hole or a semiconductor pattern. Since the number of the photolithographic process for fabricating an array substrate is determined by the number of the masks, the photolithographic process may be referred to as a mask process.
The mask process includes deposition, exposing, developing and etching processes each requiring a respective apparatus. In addition, physical and chemical fabrication steps are repeated in the mask process. As a result, the mask process for an LCD device requires a relatively high fabrication expenses.